Apparatus for sensing the quantity of reclaimed developer material

ABSTRACT

Developer material recovered in an electrostatic reproduction machine is received by a container which is mounted for displacement in accordance with the weight of the material contained therein. The container is seated on a movable platform which is subjected to a translation in accordance with the force exerted thereon. When the platform is translated in excess of a predetermined amount, a switch mechanically coupled thereto is activated. Activation of the switch is utilized to interrupt the operation of the electrostatic reproduction machine.

United States Patent 1 Nelson Oct. 1,1974

[ APPARATUS FOR SENSING THE QUANTITY OF RECLAIMED DEVELOPER MATERIAL [75] Inventor: Bruce E. Nelson, Webster, NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

22 Filed: May 14,1973

21 Appl. No.: 360,283

[52] US. Cl 355/15, 55/288, 55/429, 118/637, 222/55. [51] Int. Cl G03g 15/22 Field of Search 95/89 R; 222/D1G. l, 55;

55/28 8,429; 355/3 R, 15, 3 DD; 118/637; 354/297 M [56] References Cited UNITED STATES PATENTS Tytus 222/55 X 3,483,679 12/1969 Balbierer 355/15 UX FOREIGN PATENTS OR APPLICATIONS 11,313 6/1967 Japan 355/15 Primary Examiner-Fred L. Braun 5 7] ABSTRACT Developer material recovered in an electrostatic reproduction machine is received by a container which is mounted for displacement in accordance with the weight of the material contained therein. The container is seated on a movable platform which is subjected to a translation in accordance with the force exerted thereon. When the platform is translated in excess of a predetermined amount, a switch mechanically coupled thereto is activated. Activation of the switch is utilized to interrupt the operation of the electrostatic reproduction machine.

5 Claims, 3 Drawing Figures PATENIEU mm mm sum 20? 3 W wbx APPARATUS FOR SENSING THE QUANTITY OF RECLAIMED DEVELOPER MATERIAL This invention relates to particle reclaiming systems and, in particular, to apparatus for sensing the quantity of developer material that is deposited in recovery containers.

In the practice of xerography as described in U.S. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to selectively dissipate the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.

The electrostatic latent image may then be developed by contacting it with a finely divided electrostatically attractable developing material, such as a resinous powder. The powder is held in the image areas by the electrostatic fields on the layer. Where the field is greatest, the greatest amount of material is deposited; and where the field is least, little or no material is deposited. Thus, a viewable powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other support surface and suitably fused to thereby form a permanent print.

Present day electrostatic reproduction machines are adapted to produce copies of between 10 and 60 sheets of copy per minute and are capable of effecting solid area development. Enormous amounts of developing material are consumed duringreproducing operations requiring multiple copies and to a large extent a relatively large amount of some of this material is lost or diverted, requiring frequent additions of new material to the development system for the machines. Heretofore, there has been no effective way in which to recover residual developer material collected or otherwise made available from a developing system and which will insure the opportunity to recover the particles for later use.

In copending US. application Ser. No. 152,136 tiled on June 11, 1971, now US. Pat. No. 3,793,986, by Salvatore Latone and assigned to Xerox Corp., the assignee of the present invention, a toner reclaiming system is disclosed wherein residual developer material, removed from an electrostatic insulating surface, is deposited in receptacles and thus is provided for reuse. Briefly, the system operates by causing a flow of air to carry developer particles from a brush cleaning device in an electrostatic printing machine to a particle separation apparatus wherein the particles are separated from the air flow and conveyed to a collection manifold and thence to recovery containers. The small amount of particles remaining in the air flow are removed by a final filter element through which the air passes.

It has been recognized that the developer material contained in the recovery containers should be periodically discharged therefrom and returned to the electrostatic reproduction machine. If an operator does not attend to the evacuation of the containers, the danger exists that their capacity may be exceeded, thereby preventing further separation of the developer particles from the air flow. Consequently, the electrostatic reproduction machine would function in an enviornment that is not properly cleaned, resulting in the deleterious operation thereof. Moreover, the final filter element may be obstructed soon after the capacity of the containers is exceeded, thereby producing a substantial pressure differential thereacross. Hence. the danger of damage to the electrostatic reproduction machine and a laborious cleaning task are possibilites that should be avoided.

Therefore, it is an object of the present invention to provide apparatus for sensing the quantity of developer material that has been cleaned from an electrostatic insulating surface and deposited in recovery containers.

It is another object of this invention to provide apparatus for preventing the recovery of developer particles in excess of the capacity of the recovery containers within which said particles are deposited.

A further object of the present invention is to provide apparatus for use in an electrostatic reproduction machine for interrupting the continued operation of the machine when the amount of developer material cleaned therefrom approaches the capacity of the recovery containers within which the reclaimed material is deposited.

Various other objects and advantages of the invention will become clear from the following detailed description of an exemplary embodiment thereof, and the novel features will be particularly pointed out in connection with the appended claims.

In accordance with this invention, there is disclosed apparatus for sensing the quantity of developer material cleaned from an electrostatic insulating surface and deposited in recovery containers, said apparatus including a movable platform upon which the recovery containers are seated, said platform being subjected to a translation in accordance with the weight exerted thereon by the material contained in said containers; and switch means mechanically coupled to said platfonn and adapted to be activated when said platform is translated in excess of a predetermined amount. Activation of the switch means may be utilized to interrupt the operation of the electrostatic reproduction machine with which the recovery system is utilized. lndicating means may be coupled to the platform to provide a visual indication of the amount of developer material received by the containers.

The invention will be more clearly understood by reference to the following detailed description of an exemplary embodiment thereof in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic sectional view of an electrostatic reproduction machine incorporating the present invention therein with the processing components in section to better illustrate the enviornment for the present invention;

FIG. 2 is a schematic view of a developer material reclaiming system for the reproduction machine including the sensing apparatus of the present invention; and

FIG. 3 is a top view of the movable platform upon which the containers illustrated in FIG. 2 are seated.

For a general understanding of an electrostatic reproduction machine in which the invention may be incorporated, reference is had to FIG. 1 in which some of the various system components for the machine are schematically illustrated, and to FIG. 2 wherein the complete reclaiming system is shown. Like component parts are identified by like reference numerals throughout. As in all electrostatic systems such as a xerographic machine of the type illustrated, a light image of an original to' be reproduced is projected onto the sensitized surface of a xerographic plate to form a electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material comprising developer particles known as toner particles to form a xerographic powder image corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface to which it may be fixed by a fusing device whereby the powder image is caused to adhere permanently to the support surface.

The electrostatically attractable developing material commonly used in image developing apparatus comprises a pigmented resinous powder referred to here as ,toner" and a carrier of larger granular beads formed with steel cores coated with a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the toner powder and the granular carrier, and the toner is then attracted to the electrostatic latent image from the carrier to produce a visible powder image on the electrostatic insulating surface.

In the illustrated machine, an original D to be copied is placed upon a transparent support platen P fixedly arranged in an illumination assembly generally indicated by the reference numeral 10. While upon the platen, an illumination system flashes light rays upon the original thereby producing image rays corresponding to the informational areas on the original. The image rays are projected by means of an optical system 11 to an exposure station A for exposing the photosensitive surface of a moving xerographic plate in the form of a flexible photoconductive belt 12. In moving in the direction indicated by the arrow, prior to reaching the exposure station A, that portion of the belt being exposed is uniformly charged by a corona discharge device 13 located at a belt run extending between belt 12, a supporting roller 14, and a drive roller 15. The exposure station extends between the roller 14 and a third support roller 16.

The exposure of the belt surface to the light image selectively dissipates the electrostatic charge on the surface of the photoconductive layer in the areas struck by light, thereby forming an electrostatic latent image in image configuration corresponding to the light image projected from the original on the supporting platen. As the belt surface continues its movement, the electrostatic image passes around the roller 16 and through a developing station B located at a third run of the belt and in which there is positioned a developing apparatus generally indicated by the reference numeral 17.

The developing apparatus 17 may comprise a plurality of conventional magnetic brushes which carry developing material to the adjacent surface of the upwardly moving inclined photoconductive belt 12 in order to provide development of the electrostatic image. As the developing material is applied to the electrophotographic belt, toner particles in the development material are attracted electrostatically to the belt surface to form powder images.

The developed electrostatic image is transported by the belt 12 to a transfer station C located at a point of tangency on the belt as it moves around the drive roller l whereat a sheet of copy material is moved at a speed 5 in synchronism with the moving belt in order to accomplish transfer of the developed image. There is provided at this station a transfer roller 19 which is arranged on the frame of the machine for contacting the non-transfer side of each sheet of copy material as the same is brought into transfer engagement with the belt 12., The roller 19 is electrically biased with sufficient voltage so that a developed image on the belt 12 may be electrostatically transferred to the adjacent side of a sheet of material S as the same is brought into contact therewith.

There is also provided a sheet transport mechanism adapted to transport sheets of material seriatim from a suitable handling mechanism generally indicated by the reference numeral 23 to the developed image on the belt as the same is carried around the roller 15. A programming device operatively connected to the mechanism 23, and the aforementioned illumination system is effective to present a developed image at the transfer station C in timed sequence with the arrival of a sheet of material.

After the sheet is stripped from the belt 12, it is conveyed by a conveying system 24 into a fuser assembly generally indicated by the reference numeral 25 wherein the developed and transferred xerographic powder image on the sheet material is permanently affixed thereto. After fusing, the finished copy is discharged from the apparatus at a suitable point for collection externally of the apparatus.

The excess toner particles remaining as residue on the developed images, as well as those particles not otherwise transferred therefrom are carried by the belt 12 to a cleaning apparatus 26 positioned on the run of the belt between rollers l4, 15 adjacent the charging device 13. The cleaning device comprises a rotating brush 27, corona discharge device 28 for neutralizing charges remaining on the non-transferred toner particles and a housing 30 for supporting the brush 27 and into which the toner particles are brought to be subsequently conveyed away from the vicinity of the belt under the influence of an air stream. For this purpose, the housing 30 is in communication with the input end 31 of a duct 32 which is connected to a pumping apparatus and toner reclaiming system, generally indicated by the reference numeral 33, for removing the toner laden air from the housing 30 on a continuous basis during machine operation.

The toner reclaiming system is in the form of an air conveying system including the duct 32, an output duct 34 (clearly illustrated in FIG. 2) having a filtering device 35 as the output end for the system, pumping apparatus such as motor-driven blower or fan means 36 interposed between and in operational fluid communication with the ducts 32, 34. The blower means may be of the conventional type having an impeller (not shown) mounted for rotation within a blower housing 37 having its input end connected to the duct 32 and its output end connected to the duct 34. The operation of the blower means is such as to move air from the chamber within the brush housing 30, through the duct 32, through a separating means, through the blower housing 37, through the duct 34 and finally through a final filter element 35 and out into the surrounding atmosphere.

Positioned in the duct 32, between its input end 31 and the blower means 36, and operating as part of the air conveying system is separating means 40 which is utilized to separate the toner particles from the air flowing through thesystem. As observed from FIG. 2, the separating means 40 includes a housing 41 formed with an input fitting 42 to which the duct 32 is connected and an output fitting 43 to which a short duct 44, which is part of the main duct 32, connects the housing 41 to the input side of the blower housing 37. The separating means 40 may be of any type which is adapted to separate particles from air flow and to permit the continued flow of the clean air while moving the particles to a collecting point. It is preferred that the separator be of the centrifugal type which is constructed so that particle-laden air is set in a whirling motion to drive the particles against walls by centrifugal forces.

The toner-free air flows through the housing 41 between walls within the housing, out of the housing by way of the fitting 43 and through the duct 44 into the blower housing 37. The entire movement of air being produced by the blower means and after being directed therethrough completes its path through the duct 34 and to the filtering element 35 which is utilized in order to precipitate the remaining toner particles which may remain in the flow of air after leaving the separating means 40.

The separated toner particles are directed out of housing 41 and collect in a manifold 55 which may be suitably attached to the housing 41 as another separated chamber therein. Bellow means 61, capable of expandable and compressible deformation, are provided for supporting the recovery containers 60 in suspended relation beneath manifold 55. The recovery containers, which may comprise one or more suitable vessels, such as plastic bottles or the like, are adapted to receive the separated toner particles. Generally, the depositing of toner particles on the containers will be fairly even; that is, the containers will receive particles in substantially equal amounts. It should be appreciated that the containers are removably supported by bellow means 61 to facilitate the discharging of accumulated toner particles therefrom by an operator. The reclaimed particles may thus be reintroduced to the developing apparatus 17 through the sealing cap 72 of toner dispersing device 70 of FIG. 1. A more complete description of the toner reclaiming system, and the manner in which the system operates, is set forth in copending application Ser. No. 152,136 filed on June 1 l, l97l by Salvatore Latone.

Turning now to the subject matter of the instant invention and, more particularly, to the apparatus illus trated in FIGS. 2 and 3, the containers 60 are seated upon a movable platform generally indicated by reference numeral 62. The platform is coupled to the frame of the electrostatic reproduction machine by conven tional means 63 and is comprised of first and second portions 64 and 65, each portion being adapted to receive an equal number of containers. For the assumed example wherein the total number of containers 60 is equal to four, it is appreciated that each portion 64 and 65 receives two containers. The two portions are hingedly coupled to each other by suitable fastening means such as bolts 75 and 76 whereby each portion is independently rotatably translatable about an axis defined by the bolts. Accordingly, the platform portion 64 may be subjected to a counterclockwise translation in accordance with a force exerted thereon and platform portion 65 may be subjected to a clockwise translation in accordance with a force exerted thereon.

Switch means comprised of electromechanical switches 68 and 69 are affixed to platform portions 65 and 64, respectively. by suitable fastening means, such as the illustrated screws 80 and 81. Each switch may be a conventional microswitch having a pair of contacts adapted to be closed (or opened) upon the activation of the microswitch. Additionally. the microswitch includes a depending member adapted to be contacted by a reference member to thereby activate the microswitch. It is appreciated that the reference member may comprise the aforementioned frame of the electrostatic reproduction machine. The contacts of switch 68 are connected to conducting means 83 and 84 and the contacts of switch 69 are connected to conducting means 82 and 83. The conducting means are connected to further means for a purpose to be described. The movable platform is additionally disposed for mechanical contact with a lever 77 that is pivotable about an axis 78. The lever acts in cooperation with an index to form a visual indicating means, generally designated by the reference numberal 66, to provide an operator with an indication of the relative translation of the movable platform. The index may comprise a printed scale having appropriate calibrations thereon to designate the position of the flanged portion 67 of lever 77.

lt is here noted that the aforedescribed final filter element 35 includes a pressure sensitive switch adapted to be activated when the pressure differential thereacross exceeds a predetermined amount. A typical pressure sensitive switch is model number PSFlOOA manufactured by Fairchild Controls of Hicksville. L.l. Should the pressure differential resulting from the precipitation of particles by the final filter element 35 exceed 4 inches of water, the pressure sensitive switch is activated. The purpose of the pressure sensitive switch will become clear from the following description of the operation of the apparatus illustrated in FIGS. 2 and 3.

In operation, the weight of the developer material deposited in containers 60 effects a downward displacement of the containers which are suspended from bellow means 61. The movable platform, upon which the containers are seated, receives a corresponding force exerted thereon and is, therefore, subjected to a translation. If the toner reclaiming system described in copending application Ser. No. 152,136 filed on June 1 l, 1971 by Salvatore Latone is operating in a normal manner, each platform portion 64 and 65 will be subjected to a substantially equal translation. It may be appreciated that, as portion 65 is translated, the lever 77 that is in mechanical contact therewith, is rotated about axis 88. Thus as developer material accumulates in containers 60, platform portions 64 and 65 move downward whereby the flanged portion 67 of lever 77 moves upward. An operator may, therefore, be apprised of the amount of material that has been recovered by observing the relative position of the flanged portion 67 with respect to the calibrated index of the visual indicating means 66.

When the capacity of containers 60 is approached, the weight of the material contained therein exceeds a predetermined amount. Consequently, the movable platform 62 will have been translated beyond a predetermined limit such that the depending members of the respective switches 68 and 69 are contacted by the reference member. The switches are, therefore, activated to close (or open) their respective contacts. Signal translation paths are thus established (or interrupted) between conducting means 83 and 84 and between conducting means 82 and 83. Conducting means 82 and 84 are connected to further apparatus 90, in common relationship whereby the establishment (or interruption) of the signal translation paths energizes the further apparatus 90 to interrupt the operation of the electrostatic reproduction machine. As illustrated herein, apparatus 90 is comprised of latching relay 85 including first and second energizing coils 86 and 88 coupled to suitable sources of energizing potential 87 and 89, respectively. The latching relay further includes a single movable contact 91 adapted to selectively establish an electrical channel to conductors 92 and 93 in accordance with the particular energizing coil that has been activated. Switches 68 and 69 are electrically connected in parallel relationship such that the activation of either switch completes a series circuit including source of energizing potential 87 and energizing coil 86 whereby current flows from the source 87, through coil 86 and thence through the activated switch 68 and/or 69 to source 87. It is noted that an additional switch 95, which is the pressure sensitive switch included in the final filter element 35, is connected in parallel relationship with switch 68 and 69 and serves to complete the justmentioned series circuit upon the activation thereof. The energization of coil 86 causes contact 91 to establish a contacting relationship with conductor 92 to thereby supply control means, not shown, with a suitable potential, such as ground potential to interrupt the operation of the electrostatic reproduction machine. The contacting relationship is maintained until coil 88 of latching relay 85 is energized. This is accomplished by operating switch 94 which connects coil 88 to the source of energizing potential 89, thereby returning contact 91 to a contacting relationship with conductor 93. Hence, the aforementioned control means enablesthe machine to resume its operation. It should be noted that latching relay 85 will be reset in this manner provided the condition that caused the activation of switches 68, 69 or 95 is rectified.

If containers 60 do not receive developer material in substantially equal amounts, the translation of one platform portion will exceed the translation of the other platform portion. Nevertheless, as the capacity of the containers seated upon that one platform portion is approached, that one platform portion will be translated beyond the aforementioned predetermined limit. Thus, the depending member of a corresponding one of switches 68 and 69 is contacted by the reference member to thereby activate the switch. As a consequence thereof, the operation of the electrostatic reproduction machine is interrupted to prevent the capacity of the containers seated upon that one platform portion from being exceeded.

As noted above, the activation of switch 95 effects a latching of contact 91 to conductor 92 to thereby interrupt the operation of the electrostatic reproduction machine when the filter is obstructed by the precipitation of particles. Hence, a substantial pressure differential, e.g., a pressure differential in excess of four inches of water, is prevented from being produced across the filter, thereby guarding against the possibility of damage to the machine. The switches 68 and 69 thus cooperate with the pressure sensitive switch such that the pressure sensitive switch serves as a reserve in the event of the failure of switches 68 and 69. This obtains because the capacity of the containers would be exceeded if switches 68 and 69 fail to interrupt the operation of the electrostatic reproduction machine and the final filter would be obstructed soon after the capacity of the containers is exceeded to thereby activate the pressure sensitive switch. Moreover, it is recognized that the pressure sensitive switch is activated when the effective life of the final filter element 35 is terminated. i.e.. when the filter is substantially obstructed and no longer capable of precipitating particles.

The apparatus illustrated herein is merely exemplary and is not to be construed as unnecessarily limiting the present invention depicited in FIGS. 2 and 3. One of ordinary skill in the art will appreciate that movable platform 62 may be of unitary construction and mounted on spring biased supports. Thus, platform portions 64 and may be omitted. The movable platform may, in addition, be provided with only one switch that is activated when the platform is translated beyond a predetermined amount. Similarly, the switch may be fixedly mounted on the reference member and include a protruding member adapted to be contacted by the movable platform. The indicating means 66 may include a lever directly connected to the movable platform and adapted to be translated in the direction of translation of the movable platform. The latching relay of further apparatus 90, may include other conventional electronic circuitry. such as logic circuits or the like, to effect the interruption of the electrostatic reproduction machine and to provide a suitable indication of the cause of such interruption. Hence, the establishment (or interruption) of signal translation paths between conducting means 82 and 83 and between conducting means 83 and 84 may actuate conventional AND gate means, or the like, to control the operation of the machine.

While the invention has been particularly described with reference to an electrostatic reproduction machine, it will be obvious that this invention may be utilized with any device wherein the quantity of material deposited in a container is to be sensed. Consequently, it is apparent that the foregoing and various other changes and modifications in form and details may be made without departing from the spirit and scope of the invention. It is therefore intended that the appended claims be interpreted as including all such changes and modifications.

What is claimed is:

I. In a reproduction machine wherein residual developer material is removed from a surface for reuse, apparatus comprising:

means disposed adjacent said surface for removing said developer material from said surface; container means for receiving said developer material;

means connected to said removing means and to said container means and defining a closed passage extending therebetween;

platform means for supporting said container means,

said platform means being movably mounted so as to move in accordance with the force exerted thereon by said container means;

switch means operatively associated with said plat form means so as to be actuated when the movement of said platform means exceeds a predetermined amount; and

interrupting means responsive to the actuation of said switch means for interrupting operation of said reproduction machine when said platform means reaches a predetermined position.

2. Apparatus as set forth in claim 1, wherein said passage defining means includes a bellows.

3. Apparatus according to claim 1, which includes indicating means coupled to said platform means for providing a visual indication of the relative position of said platform means.

4. Apparatus according to claim 1, wherein said interrupting means includes reset means, and means for maintaining said reproduction machine in an inoperative mode even after said platform means moves away from said predetermined position until said reset means is actuated.

S. In a reproduction machine wherein residual developer material is removed from a surface for reuse, apparatus comprising:

container means for receiving said developer material;

platform means for supporting said container means,

said platform means being movably mounted so as to move in accordance with the force exerted thereon by said container means;

first switch means operatively associated with said platform means so as to be actuated thereby when the platform reaches a predetermined position;

a pneumatic conveying system including separating means for receiving said developer material carried by air from said surface. said separating means having a first outlet for delivering said developer material to said container means. and a second outlet for expelling air from said pneumatic conveying system;

filter means for removing any developer material in air expelled through said second outlet. the pressure across said filter means increasing with the amount of developer material removed;

second switch means operatively associated with said filter means, said second switch means being adapted to be actuated when the pressure across said filter means reaches a predetermined value; and

interrupting means responsive to said first and second switch means for interrupting the operation of said machine upon actuation of said first or said second switch means. 

1. In a reproduction machine wherein residual developer material is removed from a surface for reuse, apparatus comprising: means disposed adjacent said surface for removing said developer material from said surface; container means for receiving said developer material; means connected to said removing means and to said container means and defining a closed passage extending therebetween; platform means for supporting said container means, said platform means being movably mounted so as to move in accordance with the force exerted thereon by said container means; switch means operatively associated with said platform means so as to be actuated when the movement of said platform means exceeds a predetermined amount; and interrupting means responsive to the actuation of said switch means for interrupting operation of said reproduction machine when said platform means reaches a predetermined position.
 2. Apparatus as set forth in claim 1, wherein said passage defining means includes a bellows.
 3. Apparatus according to claim 1, which includes indicating means coupled to said platform means for providing a visual indication of the relative position of said platform means.
 4. Apparatus according to claim 1, wherein said interrupting means includes reset means, and means for maintaining said reproduction machine in an inoperative mode even after said platform means moves away from said predetermined position until said reset means is actuated.
 5. In a reproduction machine wherein residual developer material is removed from a surface for reuse, apparatus comprising: container means for receiving said developer material; platform means for supporting said container means, said platform means being movably mounted so as to move in accordance with the force exerted thereon by said container means; first switch means operatively associated with said platform means so as to be actuated thereby when the platform reaches a predetermined position; a pneumatic conveying system including separating means for receiving said developer material carried by air from said surface, said separating means having a first outlet for delivering said developer material to said container means, and a second outlet for expelling air from said pneumatic conveying system; filter means for removing any developer material in air expelled through said second outlet, the pressure across said filter means increasing with the amount of developer material removed; second switch means operatively associated with said filter means, said second switch means being adapted to be actuated when the pressure across said filter means reaches a predetermined value; and interrupting means responsive to said first and second switch means for interrupting the operation of said machine upon actuation of said first or said second switch means. 